The present invention relates generally to multichannel television systems and more particularly to an encryption/decryption subsystem particularly suited for use in subscription television distribution systems.
Subscription television distribution systems (either (1) cable or (2) over-the-air) typically provide subscribers a basic block of television channels for a basic monthly fee and one or more premium channels for an additional fee. In a typical cable system, the physical connection itself controls access to the basic channels while the premium channels are often "scrambled" (encrypted) to restrict their viewing to only subscribers who pay the additional fee.
In over-the-air systems (often called "wireless TV"), all channels are generally scrambled to prevent unauthorized reception of both basic and premium channels since the transmitted signals are available to anyone in the geographical service area. Subscribers are typically provided with a descrambler which can descramble one channel at a time. Therefore, subscriber receivers, e.g., television sets, videocassette recorders, can only be tuned to a single selected channel and features of modern television equipment that process multiple channels, e.g., "picture-in-picture", are useless.
A commonly used television encryption/scrambling method, known as "sync suppression" involves the suppression or removal of horizontal and/or vertical synchronization pulses so that a receiver is unable to properly synchronize the picture display. The subscriber's decrypter/descrambler restores, in a selected channel, the suppressed or omitted synchronization components of the received video signal so that the picture information can be properly displayed.
Sync suppression systems include both (1) baseband sync suppression and (2) RF sync suppression. In baseband systems, the signal is descrambled in baseband video after demodulation of the selected RF channel. In RF systems, descrambling is performed prior to demodulation.
U.S. Patents directed to television encryption/decryption methods and apparatus include U.S. Pat. Nos. 4,466,017; 4,503,462; 4,590,419; 4,594,609; 4,598,313; 4,602,284; 4,706,283; 4,802,214; 4,815,129; 4,817,144; 4,864,613; 4,926,477; 4,928,309; 5,058,160; and 5,093,921.